Gas turbine combustors for power generation are generally available with fuel nozzles configured for either “Dual Fuel” operation or for “Gas only” operation. “Gas Only” refers to a fuel nozzle that is restricted to providing a gaseous fuel such as natural gas for combustion in a combustion chamber of the combustor. “Dual Fuel” refers to a fuel nozzle that may be configured to provide either a liquid fuel or a gaseous fuel for combustion during operation of the combustor. Typically, the combustor will operate on gaseous fuel, however, the liquid fuel may be used as a backup or alternative fuel in the event the gaseous fuel becomes unavailable or supply is limited. In certain configurations, a gas turbine combustor may be designed to include multiple “Dual Fuel” fuel nozzles arranged annularly about a center fuel nozzle and/or a common axial centerline.
In a conventional “Dual Fuel” fuel nozzle, the liquid fuel is supplied through a liquid fuel nozzle or cartridge that extends axially within a center body portion of the fuel nozzle. The gaseous fuel is typically injected into a swirling flow of compressed air flowing through an annular passage defined between the center body and an outer burner tube, thus premixing the gaseous fuel with the compressed air before it is directed into a combustion zone defined downstream from the fuel nozzle. In particular configurations, a pilot premix nozzle or tip is disposed at a tip portion of the center body and is concentrically aligned with the liquid fuel nozzle. During operation the pilot premix nozzle may be used to provide a generally stabilized pilot flame during diffusion operation of the gas turbine even at a low fuel-to-air ratio, thus enhancing emissions performance of the combustor.
Although a gas turbine may include combustors that have “Dual Fuel” or backup fuel capability, it may not be required by the operator or in some cases the liquid fuel may not be available and/or may not be cost effective. On a gas turbine that is not required to have backup fuel capability, a gas only cartridge is provided in place of the liquid fuel nozzle, thus converting the otherwise “Dual Fuel: fuel nozzle to a “Gas Only” fuel nozzle. Purge air is directed through the gas only cartridge to keep the cartridge tip temperatures to within acceptable levels during operation of the combustor.
In particular combustors having premixed pilot nozzles, the purge air flows from the gas only cartridge and into a pilot flame provided by the premixed pilot nozzle. As a result, the purge air may decrease the stability of the pilot which may impact the performance of the combustor. Therefore an improved dual fuel premix fuel nozzle assembly, particularly one having a pilot premix nozzle and/or a gas only cartridge configured to reduce effects of purge air one the pilot flame provided by the pilot premix nozzle would be useful.